Skip to main content

Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

148                                ALASKAN GLACIER STUDIES
was the largest, but in 1905 and 1906 the stream issuing from the south margin of the glacier was many times larger than that from the northern margin.
A possible explanation of this condition is suggested by a photograph from the north side of the valley made by Gilbert in 1899. In this photograph the surface of the outwash gravel plain in front of the glacier is seen to rise perceptibly toward the south valley wall; and above a part of its surface, about a quarter of a mile from the glacier front a black area of irregular form rose above the plain (PL LXXH, B). This black elevated area was evidently lateral moraine, a continuation of the lateral moraine on the glacier, rising 'above the outwash gravel plain. Neither the black moraine nor the rising outwash gravel was present in 1905. We assume, therefore, that there was ice here in 1899 which before 1905 had so melted away as to allow the burial of the moraine, and the development of a large glacial stream on the south side.
The south stream emerged in 1905 from a large tunnel beneath an ice precipice (PL LXXlV, A), boiling out in great volume and flowing with great velocity (PL LXXIV, B) to the sea. It was so clouded with sediment as to be yellowish-brown in color, and was carrying along with it good-sized pebbles and small bowlders. Excepting near the sea, this stream did not possess any distributaries of large size during the period of our observation, differing in this respect from the northern stream, which divided and subdivided into many branches. However, that the south stream was not confined to a single channel throughout the entire season was made clear by the large numbers of branching stream courses on the outwash gravel plain, and also by the entire absence of vegetation on this plain. It was evident that during the spring melting the volume of both the south and north streams was so increased that they sent tranches in constantly-shifting, courses over all parts of the outwash gravel plain.
The Outwask Gravel Plain. It is these streams that have built the extensive gravel plain that lay between the glacier and Seal Bay in 1899 (PL LXXV, A), 1905, and 1906. In those years the streams were actively engaged in building both upward and outward, as they doubtless had been for many years before. On the seaward margin the plain stood at and below sea level, a broad, muddy, tidal flat being exposed at low tide, and shallow water extending some distance from the visible front of the delta; but at a distance of a few hundred feet from low tide line the submerged edge of the delta was reached, the front sloping abruptly to the deep water that occupied outer Seal Bay. From its tidal margin to its inner edge, at the glacier front, the surface of the outwash gravel plain, or valley train, was for the most part smooth, excepting where crossed by occupied or abandoned stream channels. The slope up to the glacier front was almost imperceptible and the elevation of the inner portion of the plain was only about 150 feet.
The inner portion of the outwash gravel plain, close by the ice front, presented peculiar and interesting phenomena. Fringing the glacier front was a broad ditch, or fosse, (Fig. 18) about 20 yards in width, terminated on each end by a series of low, hummocky ice knolls with gravel veneer, having the general appearance of groups of kame hills, with crescentic form paralleling the glacier front. One wall of this fosse was formed by the Hidden Glacier front (PL LXXV, B), which rose at an angle of about 15. The other wall, which was about 20 feet high, was a gravel-veneered ice oliff with an angle of slope greater than 80. The presence of ice in this wall of the fosse was shown in a jiumber of places where water escaped through small caves in the ice (Jiff. Ice also